The present disclosure concerns a method of manufacturing a multi-alloy aerospace component. More particularly, but not exclusively, this invention concerns a method of making an aircraft spar from two metal billets of different alloy. The invention also concerns an aerospace component so made and other related products.
Embodiments of this invention relate to a method of welding together two metal workpieces to form a bi-alloy aerospace component. In this context, an aerospace component means one that forms part of a fixed-wing aircraft, a helicopter, a missile, a satellite, a space structure or the like. The invention relates particularly, but not exclusively, to a method of welding together two large metal components to form a part or a whole of a bi-alloy spar for a wing of an aircraft and to a spar so created. It will also be appreciated that the method may be extended to make an aerospace component made from more than two different metal alloys and/or to join more than two metal workpieces and such variations are considered to be within the scope of the present invention.
In many applications it is necessary to weld together two large metal components. In many applications, it is also useful if the two components have different properties, thereby creating a structure having certain properties in one section and different properties in another section. For example, a spar in the wing of an aircraft is subject to forces in its upper section different from those in its lower section and it is therefore advantageous if the two sections are constructed from metals having different properties.
Large aircraft wings are assembled from many components by joining them together with fasteners (examples of which are bolts and rivets). Each wing typically has two main spars, a forward spar and a rear spar, both running the length of the wing. Some large wings also have a centre spar. Each spar has a length, measured in a direction from the joint of the wing and the fuselage to the wing tip, and a height, measured in a direction from the upper surface to the lower surface of the wing (typically being a maximum of the order of 1 to 2 meters in a large transport aircraft, but decreasing steadily along the length of the wing to the wing tip). The spar is relatively narrow along most of its height but the upper and lower portions are of a greater width (typically no more than 200 mm) so that the overall cross-sectional shape of the spar is usually substantially that of a ‘C’, although other shapes such as an ‘I’ are possible. The spar is strengthened at regular intervals along its length by stiffeners which are substantially the same height as the spar but have a width greater than the width of the spar away from its top and bottom portions. The stiffeners take the form of plates located transverse to the spar length. The spar may also include integrated rib-posts for facilitating the attachment of ribs that typically extend from one spar to another, in a direction transverse to the length of the spars.
A spar can be constructed from many individual components which are riveted and bolted together (known as a fabricated spar) or from one piece (from plate material, an extrusion or a forged billet) which is then machined (known as an integrally-machined spar). Fabricated spars can be formed from metal components having different properties, which are bolted together. However, fabricated spars are labour-intensive to assemble, requiring the drilling of many holes and the setting of rivets and bolts. They are also expensive because the assembly process requires sophisticated tooling, and they are heavier because of the material overlap and the rivets and bolts. They are also at greater risk of fatigue damage due to the large number of fastener holes. The advantages of integrally-machined spars are that there is no need for material overlap in the region of a join between different parts of the spar, bolts and rivets are not required (thereby reducing the overall weight) and sophisticated tooling is not needed for assembly. However, such spars are difficult to manufacture if they are to be machined from a single block of material, having differing properties in different regions.
It has been proposed (for example, see EP 1 547 720) to machine a spar from two metal components, the metals having different properties, which have been friction stir welded together. Friction stir welding is a particularly suitable method for welding together metals having different and even dissimilar properties, but has its limitations, particularly as regards the depth of welding that can be achieved, and particularly in relation to alloys developed for high-strength aerospace applications, such as 2000 series and 7000 series alloys. Currently the maximum depth of material which can be reliably welded by friction stir welding of such alloys is about 35 mm. The methods proposed in EP 1 547 720 require a channel to be machined in the two workpieces that are to be joined together so that the amount of contact measured across the width of the surfaces of the workpieces to be welded is less than the width of each component. By reducing the contact width of the surfaces it becomes feasible to friction stir weld together two workpieces with full penetration welds. A slot or channel may however be present in the finished component, which may require strengthening by means of a strap plate attached with rivets in the region of the slot or channel and/or may be prone to problems associated with increasing local stresses where a machined stiffener and/or rib post changes from a deep section to a shallow section then back to a deep section. The friction stir welding tool needed to perform the method of manufacture of EP'720 is in the form of a bobbin tool having a pair of shoulders arranged to abut opposite sides of the full-penetration joint to be welded, with a friction stir welding pin extending between the shoulders. The channel formed in the metal components must therefore be large enough to accommodate the far shoulder of the bobbin and must be of constant cross-section along its entire length. Friction stir welding with the bobbin tool must be performed along the whole length of the components, as it is not generally possible to withdraw the bobbin tool out from the welding site midway through the welding process.
The present invention seeks to mitigate one or more of the above-mentioned problems. Alternatively or additionally, the present invention seeks to provide an improved or alternative method of making a multi-alloy aerospace component.